This invention relates to control of emissions and corrosion in a contact sulfuric acid plant, and more particularly to controlling emissions and preventing corrosion by controlling acid concentrations in a high temperature sulfuric acid heat recovery absorption tower.
U.S. Pat. Nos. 4,576,813, 4,670,242 4,996,038, 5,118,490 and 5,130,112 describe the operation of a high temperature sulfuric acid absorbing tower where the heat of absorption is removed in useful form in an external acid heat exchanger, for example, a boiler producing medium pressure steam, typically at 3 to 15 bar. Steam in this pressure range can be transported and used either for process heat or to generate electricity by operation of a turbogenerator. For generation of steam at maximum feasible pressure, the high temperature absorption tower is preferably operated with countercurrent flow of acid and sulfur trioxide-containing process gas. Ordinarily, the heat recovery absorption tower serves as the interpass absorption tower for an otherwise conventional dual absorption acid plant.
U.S. Pat. Nos. 4,670,242 and 4,996,038 particularly describe an absorption tower having a condensing zone above the absorption zone for condensation of sulfuric acid vapor and absorption of residual sulfur trioxide in the gas exiting the absorption zone. Such a system is applicable to either the interpass absorption step of a dual absorption plant, or the absorption step of a single absorption plant.
It is common practice in contact acid plants to control the acid strength in an absorption tower by addition of water to the acid circulating between the tower exit and the tower inlet. In the absorption heat recovery systems described in U.S. Pat. Nos. 4,576,813 and 4,670,242, the concentration of acid exiting the absorption zone is maintained at .gtoreq.99% in order to protect the absorption acid heat exchanger from excessive corrosion at cold temperatures high enough for the generation of 3 to 15 bar steam. However, while increasing the acid concentration has a favorable effect on alloy corrosion rates, it may have an adverse effect on absorption efficiency, especially at high temperature. Since increasing temperature tends to have an adverse effect on both corrosion rates and absorption efficiency, the acid concentration at the exit of a high temperature absorption tower must be maintained within a narrow window, preferably slightly above 99% and below 100%, in order to allow recovery of absorption energy at high temperature without excessive slippage of SO.sub.3 through the absorption zone. The maximum exit acid concentration which permits efficient absorption is determined by the concentration of sulfur trioxide in the gas entering the tower, the acid temperature, and the packed height of the absorption zone. For example, in the generation of steam at about 10 bar gauge, the exit acid temperature is typically in the range of 210.degree. C. to 230.degree. C., and with reasonable packed height the maximum permissible acid concentration is in the range of 99.7% to 99.3%, respectively. If the acid concentration is significantly higher, sulfur trioxide is not efficiently absorbed and reacts with water vapor in the cooler zones of the tower to produce undesirable acid mist. Given differences in target steam pressure, heat recovery absorption systems are most preferably operated with a target exit acid concentration in the range of 99.4% to 99.6%.
As described in the aforesaid patents, the absorber is preferably operated with countercurrent flow of acid and SO.sub.3 -containing process gas through the absorption zone. This allows the acid exiting the tower to have a concentration in the desired 99.2% to 99.8% range while maintaining a driving force for mass transfer even at temperatures high enough to generate 8 to 15 bar steam in the absorption acid cooler.
Although it is important to control the acid concentration at the exit of the heat recovery absorption zone in order to balance the conflicting objectives of acceptable absorption efficiency and minimum corrosion in the absorption acid cooler, the conventional method of control has been to add water to the acid recirculating from the tower exit so as to control the concentration to the tower inlet. In the processes of U.S. Pat. Nos. 4,576,183 and 4,670,242, this is preferably accomplished by addition of water to the acid circuit between the exit of the acid cooler and the inlet of the absorption zone. However, regardless of where in this circuit the water is added, the immediate effect is on the concentration of the acid at the inlet of the tower, not the exit, so that the conventional control mode has been to control acid concentration at the inlet only, allowing the exit acid strength to float as a function of inlet acid strength, SO.sub.3 gas strength, process gas mass flow rate, mass transfer efficiency, and acid recirculation rate. The control point for inlet acid strength is adjusted as necessary to compensate for these variables and maintain the exit acid concentration within an acceptable range.
At conventional gas strength, process gas flow rate, and L/G in the absorption zone, the acid strength at the inlet of a countercurrent absorption tower is frequently controlled in the range of between about 98.3% to 98.6% in order to provide an exit acid concentration in the desired range. During operation in this range, rapid corrosion has been observed on wetted metal parts exposed to the gas stream exiting a high temperature absorption zone, for example, the acid distributor at the top of a countercurrent tower, and metal parts of a mist eliminator for removal of sulfuric acid mist from the gas. Operation at high absorption temperature in this concentration range has also been associated with excessive formation of acid mist in the absorption system. If the mist load exceeds the capacity of mist eliminators in the exit gas, acid depositing on metal surfaces of downstream equipment may cause further corrosion. Carbon steel is commonly used as the material of construction for equipment downstream of the mist eliminators such as, for example, the cold heat exchanger in which gas returning to the converter from an interpass absorption step is reheated for introduction into a downstream catalyst bed. Acid depositing on carbon steel surfaces will cause rapid corrosion.